Our team has sporadically encountered this issue but we all thought we were going a little mad.
Yesterday evening we had maintenance done on a MySQL 8.4 instance and Cloud Run started experiencing “access denied” errors. @KalleDelorean has done a great job explaining the behavior and the worrisome workarounds required to get users connected again.
Could @mcbsalceda or a Google staffer please provide an update on this issue? This does appear to be a rather serious issue that has consequences for production workflows. Downgrading to MySQL 8 feels like a shoddy solution.